Today's vehicles have powertrains that are seeking an increased number of speed ratios within the power transmission. Historically, these transmissions have had speeds ranging from two to six forward speed ratios. The transmissions employing two through five forward speed ratios have been automatic shifting planetary gear power transmissions, while transmissions providing two through six forward speed ratios have been countershaft-type transmissions. In heavy trucks, of course, there are more speed ratios available than six, many of which go as high as twenty with a large number of gear members to provide those ratios.
In planetary transmissions, two through four speeds can generally be found with two planetary gearsets and a plurality of torque-transmitting mechanisms. Five-, six-, and seven-speed power transmissions can generally be provided using three planetary gearsets and four through seven torque-transmitting mechanisms. Eight-speed planetary transmissions have been proposed using four planetary gearsets and seven torque-transmitting mechanisms.
Multi-speed countershaft transmissions generally include a synchronizer clutch or mechanical clutch for each of the forward speed ratios and one for reverse. While the forward speed ratio synchronizer clutches can generally be coupled to be double acting, it still requires a number of mechanisms to provide the speed ratios within a power transmission. For example, a four-speed countershaft transmission would require two double synchronizers for the four forward speeds and one half synchronizer or single synchronizer for the reverse speed. Thus, a great deal of mechanical devices are needed within a power transmission to provide the forward speeds required.